Beginning the review of the laws of Sukkos:[1]

It is a Mitzvah[2] upon every Jew to study[3] the laws of Sukkos prior to the festival until he is an expert in them, and knows what is to be done. One is to study the laws from the many available books dealing with the practical laws of Sukkos, or is to learn the laws from a teacher.

From when before Sukkos is one to begin studying the laws?[4] One is to begin studying the laws thirty[5] days prior to the festival. Practically, one is to begin studying the laws of Sukkos starting from the 14th of Elul.[6] 

Increasing in study two weeks prior to Sukkos:[7]  One should increase in learning these laws from two weeks before Sukkos thus following the opinion of Rav Shimon Ben Gamliel.

The 15th of Elul-the Founding of Tomchei Temimim:

On the fifteenth of Elul 5657 [1897; תרנ׳׳ז] the Tomchei Temimim Yeshiva was founded in Lubavitch.

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^[1] Admur 429:1-3; Michaber 429:1; Pesachim 6a; Tosafus ibid; Rosh Hashanah 6a; Rashi Rosh Hashanah 6a; Sukkah 9a Yerushalmi Pesachim 1:1; Rokeiach 244; Bach 429:1; M”A 429:1; Chok Yaakov 429:1-2; Yad Aaron; Hisvadyos 1982 Vol. 4 p. 2154

Background of this ruling from Admur 429:1-3:

Initial institution: The early Sages, which existed at the time that the Temple was established, instituted that the preachers should begin to publicly teach the laws of the upcoming festival, thirty days prior to the festival. Thus, from Purim [itself, starting on the 14th of Adar] and onwards they would expound on the laws of Pesach. From the fifth of Iyar and onwards they would expound on the laws of Atzeres [Shavuos], and from the 14th of Elul and onwards they would expound on the laws of the Chag [Sukkos]. [Admur 429:1; Michaber 429:1; Pesachim 6a; Tosafus ibid; Rosh Hashanah 6a; Rashi Rosh Hashanah 6a; Sukkah 9a Yerushalmi Pesachim 1:1; Rokeiach 244; Bach 429:1; M”A 429:1; Chok Yaakov 429:1-2; Yad Aaron]

The reason behind this institution: Every Jew living in Eretz Yisrael at the time of the Temple was obligated to bring three sacrifices [to the Temple] on the Festival, [the Olas Riiyah, Shalmeiy Simcha and Shalmeiy Chagigah]. [Admur ibid; Pesachim 8b; Tosafus 3b; ] Now, being that the animals to be sacrificed must be clean of blemishes and other invalidations, therefore the Sages instituted for the preachers to begin expounding on these laws thirty days prior to the holiday, in order to remind the people of the festival, so they do not forget to prepare animals which are kosher for the sacrifice. They thus had thirty days for preparation. [Admur 429:1; Beis Yosef 429; Tosafus Bechoros 57b; Mur Uketzia 429]

Does this institution apply even today? Even after the destruction of the Temple this institution remained active, as every Rebbe would teach his students the laws of the upcoming festival thirty days prior, in order so they become experts in the laws of the festival, and know that which has to be done. [Admur 429:2; Michaber C.M. 346:13; Taz 429:1; Beis Yosef 429; Tosafus Avoda Zara 5b; Bava Metzia 97a; Yerushalmi 1:1; ] In today’s generations however since the Rabbi’s no longer teach the laws to the students being that they are already all written in books, it is therefore a Mitzvah on every person to learn the laws of the festival prior to the festival until he is expert in them, and knows what is to be done. [Admur 429:3; Bach 429; Chok Yaakov 429:3; Maharil]

[2] What is the meaning of the term “Mitzvah”; is it obligatory, or voluntary? Some write that the term Mitzvah is not a complete obligation. [Chazon Ovadia Pesach p. 1; See also Kelalei Haposkim page 187 that the term Mitzvah is not an obligation.] Possibly one can say as an explanation of why learning the laws is not an absolute obligation is because since everything today is written in Sefarim one can simply look up a Halacha when it comes up, and thus there is no obligation to actually learn all the laws prior to Sukkos. Alternatively, one can explain that the institution of the Sages was only for the teachers and not the students, thus one cannot consider it an obligation for the student to learn. [See previous footnote]

^[3] Admur ibid; M”A 429:1; Majority of Poskim and Rishonim brought in Biur Halacha 429:1 “Shoalin”

Other Opinions-Asking the laws? Some Poskim write that “One asks about the laws of Pesach thirty days before” and not that “one expounds the laws of Pesach thirty days before.” [Michaber 429:1; Tur 429; Rif; Wording in Pesachim 6a and Bechoros 58] This can possibly be interpreted to mean that the Mitzvah is not to teach the laws but rather that if two people ask a question to a Rabbi, then the one who asks regarding Pesach receives precedence. And so is the ruling of the Ran Perek Kama Pesachim and Rashba Perek Daled Megillah that there is no Mitzvah to teach the laws and rather the Mitzvah is simply to give precedence to questions. Admur ibid, based on the Magen Avraham ibid, is thus negating this opinion in their wording. Furthermore, see Biur Halacha ibid who learns that majority of Poskim and Rishonim rule that one must teach the laws and that is the intent behind the word Shoalin in the Gemara ibid and used by the Michaber ibid. There is thus no dispute between the Michaber ibid and Admur in this matter, and the opinion of the Rashba and Ran are negated in face of all the other Poskim.

[4] Admur 429:1

^[5] Admur 429:1 regarding the initial Takana; Ateres Zikeinim 429 that so applies even today; Hisvadyos 1982 Vol. 4 p. 2154 mentions to begin learning the laws from the 14^th of Elul

Ruling of Admur: In 429:1 Admur writes that the institution of the Sages was to begin studying the laws starting thirty days before the festival, which regarding the laws of Sukkos is from the 14^th of Elul. However, this ruling refers to the preachers in the times of the Temple, however regarding today’s times that all the laws are written in Sefarim Admur does not write how many days before hand one is to begin the study. However, in Ateres Zikeinim 429 he writes that even in today’s times that all the laws are written in books one is to study the laws 30 days before.

^[6] Admur ibid

The calculation: Tishrei includes 14 days of learning until Sukkos which is on the 15^th. Thus, one needs to learn an additional 16 days in Elul to make up the 30 day period prior to the festival. Now since Elul is always a 29-day month, even in a leap year, therefore one must begin from the 14^th for a total of 30 days.

^[7] Based on Rebbe brought in Hamaaseh Hu Haikur regarding Pesach; See Pesachim 6a

[8] Hisvadyos 1989 4:315; Hiskashrus 995